Nocardiosis continues to be a clinically significant infection in immunosuppressed patients, including those with heart and kidney transplants and collagen vascular disease, in addition to some patients who appear immunologically "normal." Diagnosis of nocardiosis often requires invasive procedures and subsequent cultivation of the organism in the microbiology laboratory. Since effective treatment is available for this potentially lethal disease, early diagnosis is essential so that appropriate therapy can begin. A rapid, sensitive, and specific serodiagnostic assay would thus be a useful addition to the clinical evaluation of patients at high risk for nocardiosis. We have previously identified a protein with an apparent molecular weight of 55,000 daltons that appears to be a sensitive indicator of infection caused by Nocardia asteroides. As an extension of this work, the initial goal involves further steps in isolation, purification and identification of this antigen. To aid in large-scale purification of this 55,000 dalton protein, monoclonal and polyclonal antibodies will be produced for use in affinity columns. These antibodies will be useful in immunoassays to identify the antigen circulating in the blood or present in infected tissue. A well-described murine pneumonia model of nocardiosis will be used to examine the requirements for production of antibody to this antigen, as well as the kinetics of the antibody response. This murine model will also be useful in the development phase of assays for circulating and tissue-fixed antigen. Finally, the possible presence of the 55,000 dalton protein in other strains of N. asteroides as well as in N. brasiliensis and N. caviae will be determined. Knowledge of the species specificity of the 55,000 dalton protein will be important in deciding whether assays for this antigen will prove useful as an epidemiologic tool, especially in areas where N. brasiliensis and N. caviae are more prevalent than N. asteroides. Screening of other Nocardia isolates also may reveal additional species-specific antigens. Application of the previously identified immunodominant antigen, as well as other antigens to be identified, may then lead to sensitive and specific indicators of N. asteroides infection, in addition to providing valuable probes to explore the humoral immune response to N. asteroides.